1. Field of the Invention
The present invention relates to an apparatus and method for interferometrically measuring not only the distance between a test surface and a plano reference surface which are in close proximity to each other, but also the topography of the test surface. More particularly, the invention relates to apparatus which rapidly and accurately measures such distance and topography and which requires no physical contact with the test surface.
2. The Prior Art
Interferometers are generally known for determining distances and the topography of a surface under test; see, for example, C. Zanoni, "Interferometry," The Optical Industry and Systems Purchasing Directory, Book 2, pp. E-80-E-82 (1983). Interferometry relies ultimately on the measurement of phase. In traditional interferometry, the measurement of phase is derived from the geometry of the fringe pattern.
Phase measuring interferometry ascertains the phase at each point in the interference pattern by measuring the corresponding intensity variation as the overall phase is modulated.
While the prior-art fringe pattern and phase measuring interferometers are useful for many applications, there are some important measurements for which they cannot be used such as, for example, in magnetic data storage systems where it is required to measure the flying height of a slider assembly on a rapidly rotating rigid disk in order to verify the performance of the slider assembly. The flying height as used herein is the distance between the magnetic head pole and the surface of the rotating rigid disk; see, e.g., M. F. Garnier, et al., U.S. Pat. No. 3,855,625 issued Dec. 17, 1974. The flying results from the aerodynamic effects produced by the rigid disk's rotation. The flying height ranges from near contact to several hundred nanometers depending on the design of the slider. In addition to the flying height, it is desirable to measure the topography as well as the angular orientation, i.e. pitch and roll of the air bearing surfaces (ABS) of the slider, in order to assess the compliance of these parameters to the design specifications. Moreover, it is desirable to measure these aforementioned parameters quickly and automatically with minimum operator intervention. For this application, the flying height is nominally less than one-half of a wavelength of visible light.
Prior-art apparatus and methods for measuring the flying height of a slider assembly are disclosed in B. Bhushan, Tribology and Mechanics of Magnetic Storage Devices, pp. 765-797 (New York: Springer-Verlag, 1990). Prior-art methods include: visual assessment of the color bands produced by white light interferometry, and multiple wavelength interferometry, coupled with a spectral radiometer, see, e.g., A. Niagam, "A Visible Laser Interferometer for Air Bearing Separation Measurement to Submicron Accuracy," Trans. ASME, Vol. 104, pp. 60-65 (Jan. 1982); C. Lin, "Techniques for the Measurement of Air-Bearing Separation--A Review," IEEE Trans. on Magnetics, Vol. MAG-9, No. 4, pp. 673-677 (Dec. 1973); T. Ohkubo and J. Kishegami, "Accurate Measurement of Gas-Lubricated Slider Bearing Separation using Visible Laser Interferometry," Trans. ASME, Vol. 110, pp. 148-155 (Jan. 1988); and D. A. Fridge, et al., U.S. Pat. No. 4,593,368 issued Jun. 3, 1986. This technique is incorporated in commercially available products such as the line of Automatic Digital Flying Height Testers produced by Pacific Precision Laboratories, Inc. (PPL) of Chatsworth, CA. Another prior art method employs capacitive-type sensors, see, for example, G. L. Best, "Comparison of Optical and Capacitive Measurements of Slider Dynamics," IEEE Trans. on Magnetics, Vol. MAG-23, No. 5, pp. 3453-3455 (Sept. 1987). The prior-art optical techniques have generally measured the flying height using a rapidly rotating glass disk, one surface of which is a reference surface of an interferometer. White light interferometry suffers from a number of limitations; first, as the flying height gets below one-half of the shortest wavelength used, i.e., approximately 0.2 micrometers, only limited and ambiguous information is available; second, it does not take into account the wavelength dependent phase change on reflection from the air bearing surface, which affects the measurement of flying height; third, it does not lend itself to automated operation for high throughput production testing. A single slider manufacturer typically produces 200,000 to 500,000 slider assemblies per month. Similarly, the multiple wavelength interferometry technique suffers from the same limitations. The capacitive sensor approach is suitable for some laboratory testing but requires that capacitive transducers be added to the slider to be tested. For production testing this is neither practical nor cost effective. Furthermore, all of the aforementioned prior-art techniques provide poor spatial data sampling.
Commonly-owned U.S. Pat. No. 4,606,638 discloses a Fizeau interferometer for the measurement of the distance between an air bearing surface or ABS and a plano reference surface which are in close proximity to each other. The plano reference surface is a front surface polarizer. The manufacture of the front surface polarizer is very costly and any surface imperfections can cause problems at low flying heights.
Thus while prior-art fringe pattern and phase measuring interferometers are useful for some applications, they cannot satisfactorily measure the distance between a surface under test and a plano reference surface which are in close proximity to each other.